Lactose intolerance is perhaps the best-known food sensitivity in the United States and other parts of the world. It is estimated that about 70% of the world's population has a genetically controlled limited ability to digest lactose. Therefore, to help dairy maldigesters keep dairy foods in their diet, there is a growing demand for dairy food products that contain no or only low levels of lactose.
Lactase is used commercially to break down lactose in milk to produce dairy products which are suitable for people with lactose intolerance and/or have a sweeter taste. Because glucose and galactose are sweeter than lactose, lactase produces a more pleasant taste. Lactase is also used in the manufacture of ice cream. Lactose crystallises at the low temperatures of ice cream, whereas glucose and galactose stay liquid and contribute to a smoother texture. Lactase is also used in the conversion of whey into syrup. Lactase is also used for production of condensed milk.
Lactases have been isolated from a large variety of organisms, including microorganisms. Lactase is often an intracellular component of microorganisms like Kluyveromyces and Bacillus. Kluyveromyces, especially K. fragilis and K. lactis, and other fungi such as those of the genera Candida, Torula and Torulopsis, are a common source of fungal lactases, whereas B. coagulans and B circulans are well known sources for bacterial lactases. Several commercial lactase preparations derived from these organisms are available such as Lactozym® (available from Novozymes, Denmark), HA-Lactase (available from Chr. Hansen, Denmark) and Maxilact® (available from DSM, the Netherlands), all from K. lactis. All these lactases are so called neutral lactases having a pH optimum between pH 6 and pH 8. When such lactases are used in the production of, e.g., low-lactose yoghurt, the enzyme treatment will have to be done in a separate step before fermentation or rather high enzyme dosages have to be used, because their activity drops as the pH decreases during fermentation. Also, these lactases are not suitable for hydrolysis of lactose in milk performed at high temperature, which would in some cases be beneficial to keep the microbial count low and thus ensure good milk quality.
Several extracellular lactases have been described having a lower pH optimum, see, e.g., U.S. Pat. No. 5,736,374 which describes an example of such lactase, produced by Aspergillus oryzae. 
A lactase from Bifidobacterium bifidum has been described having a high transgalactosylating activity, both in the full-length form and especially when truncated from the C-terminal end (see, e.g., Jorgensen et al., 2001, Appl. Microbiol. Biotechnol. 57: 647-652 or EP Patent No. 1,283,876).
It is an object of the present invention to provide a method for production of dairy products, e.g., fermented dairy products, such as yoghurt, having a low level of lactose by using a lactase. It is also an object to provide a method for production of low-lactose beverage milk having an extended shelf-life by using a lactase, where the method gives rise to low formation of off-flavour and/or low formation of brown color as compared to known methods. Lactase to be used according to the invention should hydrolyze lactose efficiently and optimally allow for almost complete lactose hydrolysis. Especially, such lactase should have a high ratio of lactase to transgalactosylase activity. For use in the production of fermented dairy products, the lactase should be active over a broad pH range.